Slim, published in 1934, won him critical and popular acclaim as a writer. The novel was part adventure tale, part social commentary, and part-time capsule of the Great Depression, notable for its realism and its on-target portrayal of working class attitudes and language during that period. Slim continues to have an audience today and is a sought-after book, especially among utility linemen and railfans. In 1937, the novel was made into the movie Slim starring Henry Fonda, with Haines writing the screenplay. The illustrations for the hardcover of Slim were by Robert Lawson and do not appear in the paperback releases.

His next novel, High Tension (1938), was a critical success but less successful financially. His novel The Winter War (1961), about the 1877 winter campaign of the U.S. Army against the Sioux in Montana, was at the time of its publication a popular western and was awarded the Spur Award of the Western Writers of America. He also published a non-fiction book, Ultra in 1980, also based on his World War II experiences as an intelligence officer. His other books were The Honorable Rocky Slade (1955), Target (1964), and The Image (1968).

While in Europe, Haines began writing a stage play, "Command Decision", based on his 8th Air Force experiences, but was unable to sell it. Many producers rejected it, feeling it followed too closely the war's end to have popular appeal. A publisher suggested he write it as a novel, which first appeared as a four-part serial in The Atlantic Monthly between October 1946 and 1947. In January 1947, the first edition of Command Decision brought Haines critical and popular acclaim. The success of the novel revived the play, which ran for 409 performances on Broadway between October 1947 and September 1948.

In May 1947 Haines reached a deal with MGM Studios to sell it the film rights for $100,000, with an additional promise of 15% of the play's weekly gross (or approximately $300,000) if the play opened by October 1947.[2]

Command Decision as a feature film starring Clark Gable premiered in December 1948. The film and book have been called the first important work of fiction about World War II. They differ from most of the prior war-themed works from that period, which stressed the experiences of average citizens forced to go to war, in that Command Decision explores the pressures of leadership, political in-fighting, moral conflicts, and psychological effects rather than glamorizing combat, and frankly admitting the high losses in men and materiel that characterized air combat. It also takes a "warts and all" approach to the conduct of some U.S. military leaders, depicting occasional propaganda misrepresentations, personal ambitions, opportunism, and information clampdowns in the name of security. The protagonist of the story is a general who must choose between submitting to public relations demands and doing what must be done to defeat Germany. He chooses the latter and is relieved of his command for it, leading to the death of his closest friend in the process.